This invention pertains to electrochemical benzylic oxidations and more particularly to the formation of aromatic ketones thereby.
Several electrochemical benzylic oxidations to form aromatic ketones are known. The crudest are performed as a slurry of substrate in acidic or basic electrolyte. These are characterized by a multitude of products, including ring hydroxylation products and further oxidation to quinone, in addition to benzylic oxidation. Direct oxidation has been investigated in homogeneous solvent, using t-butanol as a cosolvent. Tetralin, oxidized under these conditions affords a mixture of .alpha.-tetralone, .alpha.-tetralol, and the .alpha.-(t-butyoxy)ether. The most favorable current efficiency obtained under these conditions was about 11% with .alpha.-tetralone as the product. The addition of inorganic salts to slurries of substrate and electrolyte has been investigated also. Manganous sulfate was the additive used. Russian workers reported that the yield of acetophenone from ethylbenzene rose from 42-45 to about 50-51% upon the addition of manganous sulfate to a slurry of ethylbenzene with either sulfuric acid or sodium hydroxide electrolyte. It is not clear from the context of this work if the researchers refer to electrical efficiencies as yield, although yield generally refers to the percentage of consumed substrate transformed into product, not the percentage of current that was used to oxidize product. Swiss researchers have reported permanganate ion can be regenerated electrochemically to oxidize o-toluene-sulfamide to benzoic acid sulfimide. In the oxidation of 2,4-dinitrotoluene to 2,4-dinitrobenzoic acid, chromium trioxide has proven to be the preferred additive.
There appears to be only one reference in the technical literature of the use of phase transfer catalysis in electrochemistry. L. Eberson et al., Acta. Chem. Scand. B, 29, (1975) 451-6, and Chem. Scr. 5, (1974), 47-48 reported a high yield aromatic cyanation when a biphasic aqueous-organic solvent system was used in the presence of a phase transfer catalyst (tetrabutylammonium sulfate). The phase transfer catalyst was used to ensure a high nucleophile concentration in the organic phase.